Conventional electrical power inverter devices and techniques include ferroresonant and pulse-width modulation (PWM) technologies. These two technologies are low-frequency based and require bulky electromagnetic devices sized for operation at 60 Hz.
High-frequency links have rapidly become the preferred technology in grid-connected, photovoltaic inverter applications, and are discussed in the following publications:
R. L. Steigerwald, A. Ferraro, F. G. Turnbull, "Application of Power Transistors to Residential and Intermediate Rating Photovoltaic Array Power Conditioners," to be published in the IEEE Transaction on Industry Applications. PA0 R. L. Steigerwald and R. E. Tompkins, "A Comparison of High-Frequency Link Schemes for Interfacing a DC Source to a Utility Grid," presented at the IEEE Industry Applications Society Annual Meeting, October 1982. PA0 A. Cocconi, S. Cuk and R. D. Middlebrook, "High-Frequency Isolated 4 KW Photovoltaic Inverter for Utility Interface," PCI/Motor-Con Proceedings, September 1983, pp. 39-59. PA0 W. I. Bower, T. S. Key, B. J. Petterson, "Photovoltaic Power-Conditioning Performance Evaluation, Lessons Learned," presented at the 17th IEEE Photovoltaic Specialist Conference, May 1984. PA0 T. S. Key, "Power Conditioning for Grid-Connected P. V. Systems Less than 250 KW," to be presented at the 19th Intersociety Energy Conversion Engineering Conference, August 1984. PA0 V. T. Ranganathan, P. D. Ziogas, V. R. Stefanovic, "A DC-AC Conversion Technique Using Twin Resonant High Frequency Links," presented at the IEEE Industry Applications Society Annual Meeting, October 1982. PA0 R. Goldfarb, "A New Non-Dissipative Load-Line Shaping Technique Eliminates Switching Stress in Bridge Converters," Proceedings of Powercon 8, 1981; and by PA0 W. J. Shaughnessy, "Modelling and Design of Non-Dissipative LC Snubber Networks," Proceedings of Powercon 7, 1980.
High-frequency links have the advantage of providing dc isolation and inversion without the need of 60 Hz magnetics. As a result, designs can be smaller and lighter, and exhibit a downwards cost trend based on advances in the semiconductor art.
High-frequency link inverters operate on the principle of converting dc to ac by means of a high-frequency power carrier (typically 20 kHz or above) containing a complex pattern of sidebands. This signal, when rectified and inverted, produces an ac output having a low-frequency base. The process requires several power conversion stages, none of which requires 60 Hz magnetics.
Three conversion stages are normally associated with high-frequency links: high-frequency inversion, rectification and low-frequency inversion. For the first two stages, complex snubber networks are usually applied. These networks limit semiconductor stress by shaping load lines within acceptable limits. Newly evolved snubbers can also perform this function with very low power loss by first storing transition energy in reactive components, and then releasing it to the sources. Snubber networks known to the art are disclosed by:
Resonant power conversion techniques have been recognized as an extremely efficient means of converting power, and have characteristically reduced stress on power semiconductors by switching at substantially zero current or voltages.
For example, such techniques have been discussed by I. J. Pitel, U.S. Pat. No. 4,075,476; by F. C. Schwartz and J. B. Klaasens, "A 95-Percent Efficient 1-KW DC Converter with an Internal Frequency of 50 kHz", IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol. IECI - 25, NO. 4, November 1978, pp. 326-333; and by R. L. Steigerwald, "High-Frequency Resonant Transistor DC-DC Converters" IEEE Transactions on Industrial Electronics Vol. 1E-31, No. 2, May 1984.
Two major limitations and reasons why resonant power conversion techniques have not gained increased usage are that they are not easily controlled and work well only at full power output.
With conventional 60 Hz, low frequency inverters, the power conversion circuitry is arranged to circulate periodic reactive power between the source and load to maintain a zero average real power output. This is usually done with non-controlled switches, feedback diodes, or sometimes with 60 Hz reactive components as a storage medium. In general, these inverters are characterized as being unidirectional because real power can only flow from the source to the load.
Conventional high frequency links have complicated the flow of 60 Hz reactive power. Periodic circulation of reactive power to the high frequency link occurs over many cycles, and simple feedback diodes have not been viable. A possible technique would be to double the number of switches in an opposing manner to allow bi-directional power flow. Although this method could probably work, it would increase the cost and reduce the practicality of the device.